1. Field of the Invention
This invention relates to an internal combustion engine exhaust treatment apparatus and, in particular, to a fuel fired burner for heating an exhaust stream.
2. Description of the Relevant Art
Advancement in emission technologies for internal combustion engines has resulted in significantly lowered total engine emissions. In general, automotive emissions applications employ an exhaust mounted catalytic treatment device for reduction of regulated exhaust constituents such as Hydrocarbons (HC), Carbon Monoxide (CO), and Oxides of Nitrogen (NO.sub.x) in the engine exhaust prior to its release to the atmosphere.
The catalytic treatment devices, or catalytic converters, rely on the heat of the exhaust gas to become catalytically active following a cold start. Initiation of catalytic activity occurs at the light-off temperature and is typically in the area of 400 degrees C. A substantial amount of time, on the order of 75-100 seconds or more, may be required before the engine exhaust supplies sufficient heat to the converter for the catalyst to light off. This delay in light-off of the catalyst occurs at a time when the engine is typically calibrated to run in a rich, open loop control format to insure rapid and smooth engine start-up and good vehicle performance immediately after start. Rich vehicle calibration leads to a high fuel/air ratio resulting in additional tailpipe emissions following start-up and prior to converter light-off.
In order to reduce cold-start engine emissions, it is desirable to provide an additional heat source at, or before, engine start-up to supplement exhaust heat and promote faster heating of the converter with a reduced time to optimal catalyst efficiency. In addition, it is desirable to minimize engine-out emissions prior to converter light-off by adding heat to the engine exhaust soon after its exit from the engine so as to promote the reaction of exhaust constituents such as Hydrocarbon (HC) and Carbon Monoxide (CO).
Several technologies have been considered for the preheating of catalytic converters. Electrically heated converters, which use an engine driven electrical system to heat elements in the converter, or the catalyst support itself in the case of metal supports, have been proposed. The heat output of the electrically heated units is directly related to the electrical input. In order to achieve the desired, rapid heating of the catalyst support, significant power must be supplied by the electrical system. In addition, electrically heating the catalyst support does not promote the reaction of exhaust constituents within the exhaust system upstream of the converter during the time prior to light-off when a reduction in engine-out emissions is desirable.
Burner systems, to which the present invention is directed, have also been considered for achieving rapid catalyst heating. Although not suffering from the drawbacks of electrically heated units, such systems require a burner, a fuel system, and an ignition system, along with the related complexity and packaging problems of such systems. In addition, rapid, reliable ignition of the burner, burner control, and adequate mixing of burner output with the engine exhaust have been problematic. Typically, burner based systems have not been useful for reacting the exhaust constituents within the exhaust system upstream of the converter for reducing engine-out emissions prior to catalyst light-off.